Successful rail mounting assemblies are disclosed in U.S. Pat. Nos. 6,789,740 and to 6,986,470. In these rail mounting assemblies that can be referred to as “egg” designs, the frame has a generally elongated or oval opening formed at its ends with inclined faces and four lugs symmetrically disposed at opposite ends of the frame to receive bolts for attachment of the base plate to the support structure.
The top plate is also symmetrical about the longitudinal axis and has at its ends inclined faces juxtaposed with the inclined faces of the frame and bonded, e.g. by vulcanization, to the elastomer sheet received between the juxtaposed faces and extending around the sides of the top plate and the frame. The bonding is at the inclined end faces only so that there is shear action here as well as compression to simultaneously cushion and limit relative movement of the top plate and base frame. Such track fasteners are particularly useful for vibration-sensitive locations.
In the known systems the elastomer sheet is largely unexposed and unsupported on the base frame except at the angled end faces. Even where some structure of the base frame may extend underneath the elastomer sheet other than at these angled end faces there is no bonding of the sheet to the base frame and/or top frame as the sole function of the sheet in this region is to undergo vertical compression.
Above-cited application Ser. No. 12/411,473 describes a rail-mounting assembly having a base frame formed unitarily formed with a pair of transversely spaced and longitudinally extending side members and a pair of longitudinally spaced and transversely extending end members generally bridging ends of the side members and having longitudinally inwardly directed generally vertical inner end faces. At least one web extending horizontally between the members has a substantially horizontal upper face and forms with the members at least one vertically throughgoing aperture. A top plate spaced above the base frame has a downwardly directed lower face having a portion spacedly vertically confronting the upper face of the web and a pair of respective longitudinally outwardly directed generally vertical outer end faces longitudinally spacedly horizontally confronting the inner end faces of the end members. An elastomeric body substantially fills between and is bonded to each of the outer end faces and the respective confronting inner end face. It also fills between and is bonded to the upper face of the web and the portion of the lower face confronting the upper face.
Such assemblies are fairly useful, but if the track is subjected to mixed axle traffic, dual stiffnesses would be very beneficial. If the traffic consists of transit vehicles (10-15 ton axles), high-speed passenger (20-24 ton axles) and freight (33 ton and up), instead of being locked into one stiffness that would be dictated by the heaviest axle, dual stiffness brings the choice of low stiffness (100,000 lbs/inch) for the lightest axles and high stiffness (300,000-400,000 lbs/inch) for the heaviest axles. Just such traffic is experienced in the AMTRAK North East Corridor in Trenton, namely SEPTA transit vehicles, AMTRAK Metro Liner and Conrail freight. To date such dual stiffness is unknown.